1. Field of the Invention
This invention relates to a process for extracting bitumen from oil shale, and more particularly, to a process and apparatus for extracting bitumen using supercritical solvent techniques.
2. Description of the Prior Art
Oil shale deposits are very extensive worldwide, and found in all major continents. Estimated oil shale reserves could provide hundreds or thousands of times more synthetic crude than conventional crude resources. In the United States alone, it is estimated that over seven trillion barrels of oil are contained in oil shale reserves. These reserves are found mainly in the Green River formation of Utah, Colorado and Wyoming, and in the Devonian-Mississippian Eastern Shale Deposits between the Appalachian and Rocky Mountains.
Efforts to utilize this resource fall primarily into two classes: Retorting processes and solvent extraction processes. Retorting may be further subdivided into in-situ and surface processes. Both retorting processes and existing solvent extraction processes require the addition of large amounts of heat to convert the kerogen in the oil shale to bitumen. Retorting requires high temperatures, on the order of 950.degree. F., and surface retorting utilizing gaseous heat transfer media require very large processing vessels for efficient production.
An environmental problem associated with retorting processes is a large (up to 3%) increase in volume of spent shale for disposal compared to the original shale ore. This is due in part to the high temperature vaporization of the shale oil within the shale ore particles being processed. This rapid vaporization process swells the particles and results in a net volume increase. This prohibits easy disposal of the spent shale in the original mine.
Retort reaction times and conditions must be carefully controlled to avoid converting the bitumen to lower molecular weight products and unusable residual carbon. Hydrocarbons in oil shale are in the form of kerogen. Kerogen is converted to lower molecular weight hydrocarbons ranging from methane to bitumen when heated to temperatures above 350.degree. C. Retorting processes normally operate near 500.degree. C. Extended reaction time leads to conversion of primary bitumen products to other lower molecular weight products and residual carbon. Retorting also typically produces unacceptable environmental emissions, relatively low yields of bitumen and requires heavy water usage.
The challenge in the processing of oil shale is to limit the production of gas to the amount required to fuel the process, while at the same time, minimizing the production of carbon which represents lost resource. Thus, it is desirable to recover the converted kerogen (bitumen) without the conversion to secondary lighter products. Upgrading to the desired final products is more efficient in downstream processing.
Separation of shale oil from spent shale without vaporization implies using solvent extraction. Solvent extraction is based on dissolving the converted hydrocarbon products under reaction conditions. Conversion of kerogen results in disintegration of the shale particles. The extent of this disintegration will be a primary consideration in the design of an oil shale process.
Solvent extraction methods heat the shale in the presence of a solvent to temperatures of about 100.degree. F.-1000.degree. F. and may require hydrogenation to achieve the conversion. Solvent processes generally exhibit better yields than the retorting processes, and a number of variations of the solvent process exist. However, the prior art oil shale solvent processes have problems in separating the spent shale particles from the solvent and bitumen. None use supercritical pressure to maintain the solvents in the fluid state at the elevated temperatures.
Numerous patents have been granted for inventions relating to bitumen recovery from oil shale, chiefly retorting and solvent recovery methods. Retorting processes are disclosed in U.S. Pat. Nos. 2,601,257, issued to Buchan; 3,921,193, issued to Duke; and 4,410,415, issued to Seitzer. Processes relying on means for dissolving the kerogen are disclosed in U.S. Pat. Nos. 3,017,342, issued to Bulat, et al; 3,497,005, issued to Pelopsky; 4,130,474, issued to Anthony; 4,151,067, issued to Grow; and 4,108,760, issued to Williams, et al. A process involving retorting and some aspects of solvent techniques is disclosed in U.S. Pat. No. 4,454,017, issued to Swanson.
None of the processes of the prior art have overcome the difficulties associated with the existing retorting and solvent extraction processes.